1. Field of the Invention
This invention relates to a solar cell capable of directly converting light energy to electric energy, and a method of fabricating the same.
2. Description of the Related Art
A solar cell is a semiconductor element capable of converting light energy into electric power, known types of which include p-n junction type, PIN type and Schottky type, among which the p-n junction type is widely used. It is also possible to roughly classify the solar cell into three types, based on materials composing the substrate, such as silicon crystal-base solar cell, amorphous-silicon-base solar cell and compound-semiconductor-base solar cell. The silicon-crystal-base solar cell is further classified into single-crystal-base solar cell and polycrystal-base solar cell. The silicon-crystal-base solar cell is most disseminated, because silicon crystal substrate for producing the solar cell can be fabricated in a relatively easy manner.
Output characteristics of the above-described solar cell can generally be assessed by measuring an output current-voltage curve using a solar simulator. Point Pm on the curve, giving a maximum product Ip·Vp of output current Ip and output voltage Vp is referred to as maximum output Pm, and a value obtained by dividing Pm by the total light energy incident on the solar cell (S×I: S is element area and I is intensity of irradiated light):η≡{Pm/(S×I)}×100(%)  (1)is defined as conversion efficiency η of the solar cell. For the purpose of raising the conversion efficiency η, it is important to increase short-circuit current Isc (output current value at V=0 on the current-voltage curve) or open-circuit voltage Voc (similarly output voltage value at I=0), and to shape the output current-voltage curve as being square as possible. Degree of squareness of the output current-voltage curve can generally be assessed by a fill factor (curve factor) defined by:FF≡Ipm×Vpm/(Isc×Voc)  (2)where, a value of FF closer to 1 means that the output current-voltage curve becomes more closer to an ideal square, and thereby the conversion efficiency η is raised.
For the purpose of raising the conversion efficiency η, it is important to reduce surface recombination of carriers (electrons and holes). More specifically, in a solar cell using single crystal silicon, polysilicon or the like as a substrate, minority carriers generated by the incident light such as sunray reach the p-n junction plane mainly by diffusion, extracted as majority carriers to the external through electrodes attached on the light receiving surface and back surface, generating electric energy. In this process, some of the carriers which could possibly be extracted as current may be lost through interface states which reside in the substrate surface other than electrode surfaces, and this may lower the conversion efficiency η.
Known high-efficiency solar cells therefore improve the conversion efficiency η by protecting the light receiving surface and the back surface of the semiconductor substrate, excluding contact portions with the electrodes, with an insulating film, so as to suppress recombination of the carriers at the interface between the semiconductor substrate and the individual insulating films (so-called surface passivation effect). Silicon oxide film has long been used as this sort of insulating film, but the refractive index of which is as small as 1.4 or around, and causes a slightly large reflection loss when used on the light-receiving-surface-side. For this reason, in recent years, there has been an increasing trend in using silicon nitride, having a larger refractive index, and being excellent not only in the passivation effect but also in anti-reflection effect. The silicon nitride film has conventionally been formed by the CVD (chemical vapor deposition) process such as thermal CVD, plasma CVD, photo CVD and so forth. Among these, most generally disseminated is plasma CVD.
FIG. 3 schematically shows a batch-type, parallel-plate plasma CVD apparatus, generally called a direct plasma CVD. The apparatus comprises a reaction vessel 1 equipped with an evacuation device 11, substrate holders 81 placing solar cell substrates 20 at predetermined positions in the reaction vessel 1, film forming gas introducing ducts 31, 32 introducing predetermined film-forming gases, which are reactive gases, into the reaction vessel 1, a high-frequency power source 82 generating plasma by energizing the introduced gas, and a resistance-heating heater 90 keeping a deposition atmosphere at a constant temperature. In the process of film deposition using this apparatus, predetermined film-forming gases are introduced into the reaction vessel 1 at predetermined flow rates through the film forming gas introducing ducts 31, 32, and the high-frequency power source 82 is then operated to set a high-frequency electric field. By this operation, a high-frequency discharge occurs between the substrate holders 81, thereby the film-forming gases are excited to produce a plasma, and an insulating film to be obtained is formed on the surface of the substrates 20 making use of reactions proceeded in the plasma. In an exemplary case where a silicon nitride film is formed as the insulating film, silane is introduced through the film forming gas introducing duct 31, and ammonia is introduced through the film forming gas introducing duct 32, as the film-forming gases, the both are mixed and then supplied to the reaction vessel 1, so as to produce the silicon nitride film making use of decomposition reaction and so forth of silane in the plasma.
The plasma CVD is widely applied to processes of fabricating solar cells because it can ensure a relatively high deposition rate even under a substrate temperature of relatively as low as 400° C. The process, however, raises a problem in that high-energy charged particles produced in the plasma are highly causative of damages of the deposited film or the surface of the substrates (so-called plasma damage), so that the obtained silicon nitride film tends to have a large interface state density, and consequently results in only a poor passivation effect. This is also highly affective to various characteristics of the solar cell.
There has, therefore, been proposed a CVD process making use of an ECR (electron cyclotron resonance) plasma as a method suppressed in the plasma damage. FIG. 4 schematically shows an exemplary apparatus used therefor. Unlike the conventional plasma CVD process, this method is characterized in that the surface of the substrate to be treated is placed apart from a plasma region (plasma zone) so as to make use of radical species in a separated manner, allowing this method to be referred to as “remote plasma CVD”, hereinafter. More specifically, a predetermined film-forming gas is introduced into a pre-chamber 101 at a predetermined flow rate through a film forming gas introducing duct 31, and microwaves, in place of high-frequency electric field, are applied to the pre-chamber 101 using a microwave generator 102. The microwaves raise the plasma of the film-forming gas, used also as a carrier gas, and generates reactive species. The reactive species flow into the process chamber 1, and causes chemical reactions with the other film-forming gas supplied through the film forming gas introducing duct 32, thereby an insulating film is formed on the surface of the substrate 20. In an exemplary case where a silicon nitride film is formed as the insulating film, ammonia as a film-forming gas, used also as a carrier gas, is introduced through the film forming gas introducing duct 31, and silane is introduced through the introducing duct 32, the both are mixed, so as to produce the silicon nitride film making use of ammonia decomposition reaction and so forth in the plasma. The remote plasma CVD is partially successful in reducing the plasma damage.
Whichever the plasma CVD process may be, there has, however, been only a little difference in that they were highly causative of plasma damage, and made formation of dangling bonds in the film more distinctive, so that it has been necessary to terminate the dangling bonds using a large amount of hydrogen contained in the film in view of improving the passivation effect (it has also been a natural matter of course that any known plasma CVD process was causative of inevitable incorporation of a large amount of hydrogen derived from the source gases). As a consequence, thus-obtained silicon nitride film contains hydrogen atoms to a maximum of 40 at %, and is causative of time-dependent degradation in the passivation effect under sustained irradiation of light, such as sunray, containing a large energy of ultraviolet radiation.
The conventional silicon nitride film formed by the plasma CVD process has also been shifted in the film composition thereof from the stoichiometric composition towards the silicon-excessive side to a considerably large degree, in order to obtain so-called field effect passivation. A large shift in the film composition towards the silicon-excessive side causes effluence of electrons produced by anion deficiency towards the semiconductor substrate so as to produce positive fixed charge on the cation side, and this results in band bending. This induces formation of an inversion layer in which electrons are excessive in the vicinity of the contact interface on the semiconductor substrate side, with which the passivation effect can be enhanced.
This structure, however, suffers from large drawbacks as described below.
(1) When intended for use as a back electrode, an inversion layer 112 formed in the p-type substrate 111 in the vicinity of an electrode 64 as shown in FIG. 5 tends to cause short-circuiting within an electrode surface, and this consequently results in a decrease in the generated current.
(2) The field passivation effect through formation of the inversion layer ascribable to electron effluence becomes distinctive only when the silicon nitride film is applied on the p-type layer side of the substrate. In the solar cell, carriers generated by photo-assisted excitation in p-type region and n-type region of the substrate are minority carriers, and it may be advantageous to use a p-type substrate, in which electrons having a long lifetime serve as the minority carriers, for the essential portion of the substrate contributive to light absorption, but this allows only a limited passivation effect to be exhibited on the light receiving surface side in which an n-type emitter layer is formed. The surface of the emitter layer is too high in the dopant concentration, so that the band bending can hardly occur only with an amount of fixed charge as much as residing in the silicon nitride film, and the field effect passivation is far from being expected. It can therefore be said that suppression of the plasma damage and consequent suppression of the interface states in the emitter layer hold the key for a desirable passivation. However, such desirable passivation cannot be obtained anyhow, because it is difficult to suppress damage by the general plasma CVD.
It is therefore a subject of this invention to provide a solar cell having an insulating film excellent in the passivation effect and less causative of time-dependent degradation of the passivation effect, and a method of fabricating the same.